The newly introduced, but so far little-known, Direct-Chill process, alternatively known as the Ablation Process, for shaped castings whereby an aggregate mold with a special soluble binder is removed by a fluid, such as water, has extraordinary benefits. The very high temperature gradient under which freezing occurs leads to castings of high soundness and fine internal structure. The ablation not only takes away the heat of solidification but also carries away the mold material, leaving the casting de-molded, clean, and cold, immediately ready for further processing.
One of the processes that is used for the casting of metals is investment casting, commonly known in the art as the lost pattern process. The lost pattern process is often used to create castings of complex shapes, increased dimensional accuracy (such as control of wall thickness), and/or smooth surface characteristics.
In the lost pattern process, a pattern is made and sacrificed when the molten metal is poured. A variety of pattern materials may be used, such as foam, wax, frozen mercury, or frozen water. The material to be used for the pattern depends upon the metal that is to be cast and the specific design considerations for the cast part. The known lost pattern process using a foam pattern, i.e., the lost foam process, will be described herein, although it is to be understood that the invention may be used on any known lost pattern process. The coated pattern is immersed in a loose, unbonded aggregate that is consolidated by vibration around the coated pattern. Molten metal is then poured into the pattern, displacing the pattern by the metal.
In a little more detail, the lost foam process comprises the injection of polystyrene beads into an aluminum tool, where they are expanded to fill the cavity by steam. The foamed pattern is then cooled by water cooling passages in the tooling. The tooling is then opened and the pattern ejected. The tooling has a long life because, in contrast to most other casting processes, the tooling is kept isolated from the damage caused from sand and hot metal. It only experiences the almost negligible wear from polystyrene beads. Turning to FIG. 1, the pattern 10 is removed from the die cavity and glued to a runner 12 that allows the molten metal to reach the pattern 10 upon pouring. To form a more complex pattern, several individually formed patterns may be glued together.
With reference to FIG. 2, the pattern 10 and runner 12 are dipped into a slurry of ceramic material to form a permeable coating 14 on the pattern 10. The coating 14 is dried and the pattern 10 with the runner 12 and coating 14 is lowered into a mold flask 16, as shown in FIG. 3. The flask 16 is filled with a backing material such as unbonded sand 18 that is packed around the pattern 10, often by vibration. The vibration allows the sand 18 to penetrate and support the entire pattern 10 and runner 12. A portion of the runner 12 extends to the top 20 of the flask 16 to facilitate the pouring of molten metal.
Turning to FIG. 4, a crucible 22, or similar vessel, contains molten metal (not shown) that is poured through the runner 12 and into the pattern 10. As the molten metal contacts the foam of the runner 12 and the pattern 10, the foam rapidly decomposes and is vaporized. The molten metal thus replaces the foam and the ceramic coating 14 maintains the desired shape and surface characteristics for the casting. The unbonded sand 18 supports the coating 14 to control the dimensional stability of the ceramic coating 14, and thus of the cast part.
The flask 16 is set aside to allow the cast part to cool and solidify, also known as freezing. Once cooling is complete, as FIG. 5 illustrates, the cast part 24, including a gate 26 to be trimmed, is removed from the sand 18. After solidification, the casting is easily separated from the loose unbonded backing aggregate, and is cleaned from adhering coating. This can be done either by extracting the part 24 from the sand 18 or dumping the sand 18 out of the flask 16. The sand 18 is typically reclaimed and re-used. The ceramic coating 14 (referring back to FIG. 4) is removed from the cast part 24 by tumbling or another operation known to those skilled in the art.
This process is used for a wide variety of castings. In particular the advantages of this known process include:                (i) The avoidance of the manufacture of cores (the major disadvantage of cores being the rapid wear of core boxes and other tooling). This activity is replace by the manufacture of Styrofoam patterns, with greatly reduced wear of tools and consequently much longer tool life;        (ii) the absence of parting lines on the product (although it is hoped that the glue bead lines will eventually be solved, eliminating the last trace of this problem);        (iii) possibility of zero draft;        (iv) capable of production of cast parts of great complexity;        (v) potential for excellent control of wall thickness; and,        (vi) use of unbonded aggregate comprising the main body of the mold.        
In addition to its excellent unique features, it is unfortunate that the lost foam process has a number of well-known disadvantages. These include:                (i) The tooling is highly complex and therefore expensive. Complex parts such as cylinder heads and blocks can only be made by specialist toolmakers. For these reasons the process is generally limited to those parts requiring long production runs;        (ii) good filling system designs are not easily employed, partly because the pattern needs the strength to withstand handling and dipping;        (iii) the pattern is relatively flimsy and is easily distorted during the pouring of the backing aggregate;        (iv) black fume is evolved from the foam on pouring;        (v) the backing aggregate (sometimes silica sand or other non-silica aggregate) becomes gradually contaminated with decomposition products of styrene, making the aggregate sticky and, probably, to some extent toxic;        (vi) the metal is cooled considerably by the necessity to vaporize the foam, leading to the necessity for very high pouring temperatures;        (vii) the casting usually has a significant content of defects arising from the high hydrogen content (one of the decomposition products of the organic foam); and        (viii) fold defects are the most serious faults. These arise because of difficulty in controlling the filling in a reproducible way. Even during counter-gravity filling (such as that disclosed in U.S. Pat. No. 6,103,182) of lost foam molds, the progress of the advance of the liquid metal is not usually smooth or predictable. This is because the density of the foam is not easily controlled, so that the melt advances more rapidly through less dense regions, often falling back onto other regions, and thereby enfolding defects.        
Some of these problems are reduced in a number of variants of the process. These include:                (i) Counter-gravity filling of lost foam molds which, despite not being perfect as noted above, still gives superior castings to those produced by gravity pouring;        (ii) hydrogen porosity has been reduced by some casters by the application of pressure after pouring;        (iii) many of the quality problems with lost foam castings arise because of the degradation of the foam during casting, in which form the process is sometimes known as the ‘Full Mold’ Process. One of the most effective ways to avoid a significant number of the above disadvantages clearly results from the elimination of the foam prior to casting. This is, of course, an expensive step, but is justifiable for products in which contamination by the products of degradation of the foam is not acceptable, as, for instance, is the case for the casting of low carbon steels that would otherwise be contaminated with carbon. The prior elimination of the foam is one of the variants of the Replicast Process developed in the UK.        
Still, the foam patterns are relatively weak and must withstand handling and being dipped in the ceramic slurry. This causes designs of patterns to focus on strength rather than better filling, thereby sacrificing optimum casting process characteristics. The weakness of foam patterns also often leads to distortion of the patterns when the backing material is poured around the pattern in the flask. Such weakness of the patterns leads to a need for a coating that may lend more structural support to the patterns.
Other disadvantages of the lost foam casting process are associated with the slow cooling of the cast metal. As mentioned above, after the molten metal is poured into the mold, the mold is typically set aside until enough heat has been lost from the metal so that it has solidified, whereupon the casting is removed from the mold.
The sand that serves as the backing material in lost foam casting is most commonly silica. However, silica experiences an undesirable transition from alpha quartz to beta quartz at about 570 degrees Celsius (° C.), or 1,058 degrees Fahrenheit (° F.). In addition, a silica backing aggregate typically does not allow rapid cooling of the molten metal due to its relatively low thermal conductivity.
Rapid cooling of the molten metal is often desirable, as it is known in the art that with such cooling the mechanical properties of the casting are improved. Moreover, rapid cooling allows the retention of more of the alloying elements in solution, thereby introducing the possibility of eliminating subsequent solution treatment, which saves time and expense. The elimination of solution treatment prevents the quench that typically follows, removing the problems of distortion and residual stress in the casting that are caused by the quench.
As a result, it is desirable to develop a lost foam casting process and related apparatus that provide the advantages of increased structural support of the pattern and more rapid solidification of the cast metal.